1. Field of the Invention
The present invention relates to an image processing method and an image processing apparatus for carrying out sharpness processing on image data at a desired resolution out of multiple-resolution image data obtained by converting image data into multiple resolution spaces, and also to a computer-readable recording medium storing a program for causing a computer to execute the image processing method.
2. Description of the Related Art
Various kinds of formats for storing image data, such as JPEG, GIF, and TIFF have been known. According to another format proposed recently, image data are stored in a hierarchy based on resolution or density resolution and data at each level of the hierarchy (hierarchical data) are stored after coding and compression thereof. More specifically, according to this storage format, image data are decomposed into hierarchical data according to multiple resolutions or density resolutions through wavelet transform or the like carried out thereon, and the hierarchical data at each resolution or density resolution are coded and compressed according to the hierarchy and saved in a file.
This storage format has the following characteristics:
(1) Unlike a DCT (Discrete Cosine Transform) method used in the conventional JPEG format, no artifacts such as block distortion are created, since image data are not processed in blocks.
(2) Only resolution information which is necessary upon transfer of image data needs to be transferred, since the image data have been coded hierarchically. This enables efficient image transfer.
(3) Various kinds of image processing such as frequency enhancing processing can be carried out comparatively easily, since image data are decomposed into data at multiple resolutions or multiple density resolutions.
(4) Simultaneous decomposition of color space and frequency according to multiple-resolution analysis is possible. In a low frequency range wherein coding efficiency is greatly affected, an orthogonal transform is carried out in a wide range, whereas in a high frequency range, an orthogonal transform is possible in a narrow range. Therefore, even when quantization noise near an edge of an image is generated, spread of the noise in a color space can be suppressed. As a result, the noise is not easy to perceive.
Like a FlashPix file proposed by Eastman Kodak Co., a file format enabling storage of data sets having different characteristics in a file has also been proposed. Hierarchical data decomposed into multiple resolutions or density resolutions can be stored in such a file according to the FlashPix standard.
Meanwhile, in order to enhance sharpness of images, sharpness processing may be carried out on image data according to the following equation for unsharp masking:
Sproc=Sorg+xcex2xc2x7(Sorgxe2x88x92Sus)
where Sproc: image data having been processed
Sorg: original image data
Sus: unsharp masking image data and
xcex2enhancement factor (1, for example).
Image data are transferred between various image servers or clients via a network. Therefore, information regarding sharpness enhancement processing (for example, the enhancement factor xcex2 in the above equation) is attached to image data and the image data attached with the information are transferred. By attaching the information regarding sharpness processing (hereinafter called sharpness information) to the image data, a client which has received the image data can carry out appropriate sharpness processing on the image data, based on the sharpness information.
It is possible to attach the sharpness information to the image data converted into multiple resolution spaces. However, clients request only image data at an intermediate resolution in some cases. This is because image data at a highest resolution are not necessary for display on a comparatively low resolution monitor, and transfer of the image data at the highest resolution is time-consuming when a network is busy. In this case, since the sharpness information is transferred with the image data at resolutions up to the intermediate resolution, the client can carry out sharpness processing on the intermediate resolution data based on the sharpness information. However, the sharpness information has been set to correspond to the image data at the highest resolution, and the image data at the intermediate resolution do not have a high frequency component. Therefore, when the sharpness processing is carried out on the image data at the intermediate resolution based on the sharpness information, an image obtained thereby becomes more blurred than an image is intended to be. Furthermore, the sharpness information may have been set to correspond to not only the image data at the highest resolution but also image data at a predetermined resolution (xc2xd of the highest resolution, for example). In this case, when the sharpness processing is carried out based on the sharpness information on image data at a resolution other than the predetermined resolution, high frequency component information may be lost or become too large in some cases. Therefore, sharpness of the image data to be obtained does not become appropriate.
The present invention has been created based on consideration of the above problems. An object of the present invention is to provide an image processing method and an image processing apparatus for carrying out appropriate sharpness processing on image data at a resolution other than a predetermined resolution for which sharpness information has been set out of multiple-resolution image data, and also to provide a computer-readable recording medium storing a program to cause a computer to execute the image processing method.
An image processing method of the present invention is an image processing method of carrying out sharpness processing on image data at a desired resolution other than a predetermined resolution out of multiple-resolution image data obtained by converting image data into multiple resolution spaces and attached with sharpness information regarding sharpness processing to be carried out on the multiple-resolution image data upon reproduction of the image data at the predetermined resolution. The image processing method comprises the steps of:
determining sharpness processing to be carried out on the image data at the desired resolution, based on the desired resolution and the sharpness information; and
obtaining processed image data by carrying out the determined sharpness processing on the image data at the desired resolution.
The phrase xe2x80x9cattached with sharpness informationxe2x80x9d means not only recording sharpness information in a header or the like of a file storing the multiple-resolution image data, but also recording the sharpness information in a file separate from a file storing the multiple-resolution image data and correlating the file storing the image data to the file storing the sharpness information.
As the xe2x80x9csharpness informationxe2x80x9d, not only an enhancement factor in unsharp masking, but also information representing a frequency characteristic of the image data, such as a function representing the enhancement factor and information regarding an image generating apparatus from which the image data have been obtained or an image reproducing apparatus for reproducing the processed image data, can be included.
An image processing apparatus of the present invention is to carry out the image processing method of the present invention, and comprises:
determination means for determining sharpness processing to be carried out on image data at the desired resolution, based on the desired resolution and the sharpness information; and
sharpness processing means for obtaining processed image data by carrying out the determined sharpness processing on the image data at the desired resolution.
The predetermined resolution may be a highest resolution of the image data.
It is preferable for the sharpness information to include a characteristic value of a filter for converting the image data into multiple resolution spaces.
It is preferable for the sharpness processing to cause sharpness at a predetermined frequency included in a frequency band of 0.5xcx9c2.0 cycles/mm of an image reproduced from the processed image data to approximately agree with sharpness at the predetermined frequency of the image data at the predetermined resolution on which the sharpness processing is carried out based on the sharpness information.
Furthermore, the image processing method of the present invention may be provided as a program for causing a computer to execute the image processing method recorded in a computer-readable recording medium.
According to the present invention, based on a desired resolution and the sharpness information attached to image data at multiple resolutions, sharpness processing to be carried out on the image data at the desired resolution is determined and the determined sharpness processing is carried out on the image data at the desired resolution. Therefore, desired sharpness processing can be carried out on image data at a resolution other than the predetermined one. As a result, by reproducing the processed image data, an image having appropriate sharpness and creating no visual discomfort can be obtained.